1. Technical Field of the Invention
The present invention relates to an epitaxial substrate for a semiconductor element, and particularly to an epitaxial substrate having a multilayer structure made of a group-III nitride semiconductor.
2. Description of Related Art
A nitride semiconductor is attracting attention as a semiconductor material for a next-generation high-frequency/high-power device, because the nitride semiconductor has a high breakdown electric field and a high saturation electron velocity. For example, an HEMT (high electron mobility transistor) device in which a barrier layer made of AlGaN and a channel layer made of GaN are laminated takes advantage of the feature that causes a high-concentration two-dimensional electron gas (2 DEG) to occur in a lamination interface (hetero interface) due to the large polarization effect (a spontaneous polarization effect and a piezo polarization effect) specific to a nitride material (for example, see Non-Patent Document 1).
In some cases, a single crystal (a different kind single crystal) having a composition different from that of a group-III nitride, such as silicon and SiC, is used as a base substrate of an HEMT-device substrate. In this case, a buffer layer such as a strained-superlattice layer or a low-temperature growth buffer layer is generally formed as an initially-grown layer on the base substrate. Accordingly, a configuration in which a barrier layer, a channel layer, and a buffer layer are epitaxially formed on a base substrate is the most basic configuration of the HEMT-device substrate including a base substrate made of a different kind single crystal. Additionally, a spacer layer having a thickness of about 1 nm may be sometimes provided between the barrier layer and the channel layer, for the purpose of facilitating a spatial confinement of the two-dimensional electron gas. The spacer layer is made of, for example, AlN. Moreover, a cap layer made of, for example, an n-type GaN layer or a superlattice layer may be sometimes formed on the barrier layer, for the purpose of controlling the energy level at the most superficial surface of the HEMT-device substrate and improving contact characteristics of contact with an electrode.
It is known that, in a case where a nitride HEMT device has the most general configuration in which a channel layer is made of GaN and a barrier layer is made of AlGaN, the concentration of a two-dimensional electron gas existing in an HEMT-device substrate increases as the AlN mole fraction in AlGaN of the barrier layer increases (for example, see Non-Patent Document 2). If the concentration of the two-dimensional electron gas can be considerably increased, the controllable current density of the HEMT device, that is, the power density that can be handled, would be considerably improved.
Also attracting attention is an HEMT device having a structure with reduced strain, such as an HEMT device in which a channel layer is made of GaN and a barrier layer is made of InAlN, in which the dependence on a piezo polarization effect is small and only a spontaneous polarization is used to generate a two-dimensional electron gas with a high concentration (for example, see Non-Patent Document 3).
In a case of forming an HEMT device having the InAlN/GaN structure in which a channel layer is made of GaN and a barrier layer is made of InAlN, a cap layer including InAlGaN that contains at least Ga, such as GaN or AlGaN, is sometimes formed on the barrier layer, for the purpose of current collapse suppression. In this case, depending on conditions under which the cap layer is formed, a Ga element may diffuse within an InAlN layer that is the barrier layer, which consequently causes an electron state of an epitaxial film to vary so that device characteristics are deteriorated.
Also, in a case of forming an HEMT device having the InAlN/GaN structure, a nitride cap layer containing Mg is sometimes formed on the barrier layer, for the purpose of establishing a PN junction with an InAlN layer, as a gate electrode structure, instead of a Schottky junction. In this case, depending on conditions under which the cap layer is formed, the Mg element doped as an acceptor diffuses within the InAlN layer that is the barrier layer, so that the PN junction cannot successfully obtained.